Communication systems employing high-speed serial transmission are well-known in the prior art. Generally in communication systems, parallel data signals are output from a data generation portion to a transmitter, and in the transmitter the parallel data signals are converted into serial data signals and transmitted to a high-speed serial transmission line. At this time, an operation clock of the transmitter synchronizes parallel data signals from the data generation portion and is transmitted. And, this operation clock is a clock obtained by spread-spectrum (SS) modulation of the frequency of a reference clock in a clock generation portion, and is output to the data generation portion. Such a communication system is for example described in Non-patent Reference 1.
Here, increasing the frequency modulation factor resulting from spread-spectrum modulation of the operation clock is effective for reducing EMI (Electro-Magnetic Interference). Hence in order to lower EMI in communication systems, it is preferable that the modulation factor of the clock generated by the clock generation portion be high.